Azimuthally modulated scattering device

ABSTRACT

Optical elements with anisotropic, patterned surface relief microstructures in which information is encoded in the distribution of the orientation of different zones. From the analysis of the distribution of the light scattered from the element, the orientation distribution in the element and therefore the encoded information can be evaluated. The elements are particularly useful for securing documents and articles against counterfeiting and falsification.

TECHNICAL FIELD

The invention relates to optical elements with anisotropic, patternedsurface relief microstructures in which information is encoded in thedistribution of the orientation of different zones. The elementsaccording to the invention are particularly useful for securingdocuments and articles against counterfeiting and falsification.

BACKGROUND OF THE INVENTION

There is a constant need for novel distinctive features in opticalsecurity elements for forgery protection. Depending on the toolsrequired for verification, security devices are categorized in threesecurity levels. First level devices have overt features, which can beverified by people, for example by optical recognition. There is astrong need for first level features, in particular for banknotes,credit cards or other value documents, as they can be fast and easilyverified without any additional tool. Second level features aresemi-covert features such as microprints or uv-activated features whichneed additional tools for verification, for example throughvisualization. Third level security devices have covert features, whichneed detection systems for checking the authenticity. Optical elementscomprising a combination of different levels of security offer anincreased protection against forgery.

Optically variable devices (OVD), such as holograms and kinegrams, arewell known first level optical security features. In addition, there arefirst level optical elements, which are based on anisotropic opticalscattering.

WO2007/131375 discloses optical elements using anisotropic scattering todisplay high resolution optical information, for example in the form ofimages, photographs, graphics or lettering, with a pronouncedpositive-negative switch upon tilting or rotating the element, therebyproducing an obvious contrast reversal of an image. The opticalinformation may appear as black and white or colored. Because of thenon-periodic, anisotropic surface relief structure, which causes theinteraction with the incident light, the images appear without thetypical rainbow colors known from holograms or kinegrams. Therefore, theoptical feature and the instruction how to verify it can easily bedescribed to a layman.

U.S. Pat. No. 7,710,652B2 discloses anisotropic scattering structurescomprising a plurality of grating lines, which, for example, can becreated by electron beam lithography. The grating lines have a randomvariation of the spacing, curvature, orientation or profile.

There is still a constant need for novel distinctive features in opticalsecurity elements for forgery protection.

SUMMARY OF THE INVENTION

An object of the present invention is, therefore, to provide an opticalelement with novel security features as well as correspondingverification methods. A further object is to provide methods formanufacturing such optical elements.

According to a first aspect of the invention an optical element isprovided comprising a region with a patterned anisotropic surface reliefmicrostructure, which has zones with different anisotropy directionsθ_(i), wherein information is encoded in the distribution of zones withthe different anisotropy directions.

The distribution of zones with the different anisotropy directions canbe characterized by f_(i)(θ_(i)), which is the fraction of the areas ofzones with the anisotropy direction along θ_(i) with regard to the totalarea with anisotropic surface relief microstructure in that region.

The region comprising the different zones with the patterned anisotropicsurface relief microstructure may also comprise areas withoutanisotropic surface relief microstructure. For the calculation of thefractions f_(i)(θ_(i)) those areas are not taken into account.

When observed by naked eye, the region comprising the pattern may appearuniform and an observer will not be able to determine the informationwithout additional evaluation tools.

The optical effect of the elements according to the invention is basedon anisotropic light scattering. FIGS. 1.1 and 1.2 illustrate thedifference between isotropic and anisotropic light scattering.

Scattering at an isotropic scattering surface is such that no azimuthaldirection is preferred. As indicated in FIG. 1.1, collimated incominglight 1 is redirected at the scattering surface 2 into new outgoingdirections 3 with a characteristic axial-symmetric output lightdistribution and a characteristic divergence angle 4.

In case of an anisotropic scattering surface the light is scattered intopreferred azimuthal directions. In FIG. 1.2, collimated incoming light 1impinges on an anisotropically scattering surface 5 and is redirectedinto new outgoing directions 6 with a characteristic output lightdistribution 7.

In the context of the present invention, the term anisotropy directionshall mean a local symmetry axis within the plane of a layer, forexample the direction along grooves or valleys of a microstructure.

If a surface comprises a pattern of anisotropic structures with locallydiffering anisotropy directions, like the directions 10, 11 in FIG. 2,then the individual zones of the pattern scatter incoming light intodifferent directions.

A region in which information is encoded in the distribution of zoneswith different anisotropy directions may have any form.

The pattern encoding information by the orientation distribution can bethe background of an OVD scattering element or may be the fill patternof characters, numbers, logos or any other image.

The information encoded in the distribution of zones with the differentanisotropy directions may be the only information present in an opticalelement. However, the pattern encoding the information by theorientation distribution may be combined with other security features.For example, the regions comprising the pattern which encodesinformation by the orientation distribution may be an area inside oroutside a structure of another security feature, for example of an OVDfeature, such as a hologram, a kinegram or a scattering devicedisplaying any other information. It could be applied as a background ofan OVD scattering element or may be the filling pattern of characters,numbers, logos or any other image.

As an example of a preferred embodiment of the invention, FIG. 3 showsan element 20, which displays a star 22 on a background 21. Thebackground comprises an anisotropic surface relief microstructurepattern according to the invention, which encodes information by thedistribution of zones with the different anisotropy directions.Macroscopically, the background looks uniform and it is possible toevaluate the encoded information at any point of the background. Thestar is visible because of a different optical effect. Any opticaleffect that makes the star visible could be used. In the simplest formthe star is printed, but it could also comprise a diffractive orscattering structure. For example, the star may comprise an anisotropicsurface relief microstructure, which may be uniform. In a preferredembodiment of the invention, both the background as well as the areainside a displayed information comprises an anisotropic surface reliefmicrostructure pattern according to the invention, which both encodeinformation by the distribution of zones with the different anisotropydirections.

An element according to the invention is preferably applied as securityfeature in banknotes or other value documents. For example it may beapplied as a stripe or a patch.

According to a second aspect of the invention a method is provided forevaluating the information encoded in the orientation distribution. Themethod comprises

-   -   detection of the spatial light distribution scattered from an        element comprising a region with a patterned anisotropic surface        relief microstructure, which has zones with different anisotropy        directions θ_(i), wherein information is encoded in the        distribution of zones with the different anisotropy directions,    -   evaluation of the fractions f_(i)(θ_(i)) from the measured        spatial light distribution, wherein f_(i)(θ_(i)) is the fraction        of the areas of zones with the anisotropy direction along θ_(i)        with regard to the total area with anisotropic surface relief        microstructure in that region.

Preferably, the dimension of the smallest zones of the pattern is muchsmaller than the size of the measurement spot. Any position is thenmacroscopically equivalent, which makes the element convenient formachine-readable verification.

Elements according to the invention allow to design a precise scatteringpattern where the lobes appear at a certain azimuthal angle, with acertain relative scattering intensity. Such scattering pattern can beused to verify the authenticity of the element onto which it isattached. The analysed signal can be the total 3D scattering pattern ora 2D cut.

The intensity of the light scattered into the different directions isproportional to the fraction of the zones with the correspondingorientation direction. Accordingly, the information encoded in theorientation distribution results in a characteristic distribution of thescattered light. From the analysis of the distribution of the scatteredlight, the orientation distribution in the element according to theinvention and therefore the encoded information can be evaluated.

There are different methods to evaluate the spatial pattern of thescattered light. A preferred method is by means of conoscopy imaging.Conoscope measurement systems are commercially available and providesophisticated evaluation algorithms. This allows fast and accuratemeasurement. Alternatively, a simple evaluation setup is the use offisheye lens conoscopy which uses a CCD camera coupled to a fisheyelens, as described by A. Alshomrany and N. A. Clark in Liquid Crystals,Vol. 42, No. 3, 271-287 (2015).

Preferably, method for evaluating the information encoded in theorientation distribution comprises measurement of the spatial lightdistribution using a conoscopy imaging system.

According to a third aspect of the invention there is provided a methodfor the manufacturing of an element according to the first aspect of theinvention. The method for manufacturing of the optical element comprisesusing an encoding algorithm to generate a pattern of an orientationdistribution, which represents the information to be encoded. The methodfurther comprises the manufacturing of a patterned anisotropic surfacerelief microstructure, which has zones with different anisotropydirections θ_(i), according to the pattern that has been calculated inorder to encode the information.

Methods for the production of patterned anisotropic surface reliefmicrostructures are well known to a skilled person. In particular, themethods may be used which are disclosed in the above cited documentsWO2007/131375 U.S. Pat. No. 7,710,652B2 or in WO-A-01/29148.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1.1 is an illustration of light reflection at an isotropicallystructured surface.

FIG. 1.2 is similar to FIG. 1.1, but illustrates the characteristicoutput light distribution from a reflection at an anisotropicallyscattering surface.

FIG. 2 illustrates pixels with differing anisotropy directionorientation.

FIG. 3 shows an optical element according to the invention whichcombines a first level and a third level security feature.

FIG. 4 shows patterned anisotropic surface relief microstructuresaccording to the invention manufactured on a silicon wafer.

FIG. 5 illustrates the geometry used for the angular dependentmeasurement of the scattered light intensity.

FIG. 6a shows the form of the pattern used in example 1. FIG. 6b showsthe related conoscopic measurement.

FIG. 7a shows the form of the pattern used in example 2. FIG. 7b showsthe related conoscopic measurement.

FIG. 8a shows the form of the pattern used in example 3. FIG. 8b showsthe related conoscopic measurement.

FIG. 9a shows the form of the pattern used in example 4. FIG. 9b showsthe related conoscopic measurement.

DETAILED DESCRIPTION OF THE INVENTION

Preferably, an average diagonal of the smallest zones of a pattern aresmaller than 1 mm, more preferred smaller than 0.3 mm and most preferredsmaller than 0.1 mm.

Preferably the zones with different orientation of the anisotropy axisare randomly distributed.

The minimum size of an area of a region with a defined ratio of thezones oriented in the different directions depends on the opticaldetection system. Preferably, an average diagonal or diameter of aregion is larger than 0.5 mm, more preferred larger than 1 mm and mostpreferred larger than 2 mm.

The number of different directions in a pattern of an element accordingto the invention is 2 or higher. In principle an element may have anynumber of directions. However, the measurement system and the evaluationalgorithm may have limitations in resolution. Therefore, it may bereasonable to use less than 10 different orientation directions. Mostpreferred are pattern with 3, 4 and 5 different orientation directions.

The number of different information that can be encoded in a patterndepends on the number of different directions and the number ofintensity levels that can be distinguished by the measurement andevaluation system. As an example, if a pattern has 3 differentdirections and the evaluation system allows to distinguish 20 differentintensity levels associated with each of the directions than 8000different information can be encoded in a single pattern.

Preferably, the anisotropic surface relief microstructure in the patternencoding information by the orientation distribution is non-periodic.

Preferably, there are areas wherein the average structure depth of thenon-periodic, anisotropic surface relief microstructure is larger than60 nm, more preferably, the average structure depth of themicrostructure is larger than 90 nm. For the generation of colors, theaverage structure depth of the microstructure is preferably larger than180 nm, more preferred larger than 300 nm and most preferred larger than400 nm. Preferred ranges of the average structure depth for providingdistinctive colors are 180 nm to 230 nm, 240 nm to 280 nm, 290 nm to 345nm, 365 nm to 380 nm and 430 nm to 600 nm.

Contrary to a periodic structure, which repeats itself after a certaininterval, and which is therefore predictable once the structure of aperiod is known, as the surface profile of a non-periodic structurecannot be predicted at a distance from a known part of the structure.For determination of a surface profile being non-periodic theautocorrelation function and a related autocorrelation length can beused. The autocorrelation function of a surface profile can beunderstood as a measure for the predictability of the surface profilefor two spatially separated points by a distance x in the plane.

The autocorrelation function AC(x) of a function P(x), such as thesurface relief microstructure profile, is defined as

AC(x)=∫P(x′)·P(x′+x)·dx′

For a non-periodic or non-deterministic surface profile, theautocorrelation function decays rapidly with increasing x. On the otherhand, for a deterministic surface profile found for instance in agrating, the autocorrelation function is modulated with a periodicfunction but the amplitude does not decay.

With the help of the autocorrelation function, a single characteristicnumber, an autocorrelation length L, can be defined. It is the lengthfor which the envelope of the autocorrelation function decays to acertain threshold value. For the present purpose, a threshold value of10% of AC(x=0) proved to be suitable.

In the context of the present invention it is preferred that anon-periodic, anisotropic surface relief microstructure has at least inone direction an averaged one-dimensional autocorrelation function AC(x)that has an envelope, which decays to 10% of the AC at x=0 within anautocorrelation length, wherein the autocorrelation length is smallerthan three times an average lateral distance between adjacenttransitions of top and bottom regions, such as hills and valleys.Preferably, the one direction is perpendicular to the anisotropydirection. Preferably, the anisotropic surface relief microstructure isalso modulated along the anisotropy direction y, such that the envelopeof an averaged autocorrelation function AC(y) decays to 10% of the AC aty=0 within an autocorrelation length, wherein the autocorrelation lengthis smaller than three times an average lateral distance between adjacenttransitions of top and bottom regions along the anisotropy direction.

More preferred are surface relief microstructures, wherein theautocorrelation length is smaller than two times an average lateraldistance between adjacent transitions of top and bottom regions. Evenmore preferred are surface relief microstructures, wherein theautocorrelation length is smaller than one average lateral distancebetween adjacent transitions of top and bottom regions.

Preferably, the autocorrelation length (L) is greater than one hundredthaverage lateral distance between adjacent transitions of top and bottomregions.

There are different known methods, which can be used to generatenon-periodic, anisotropic surface relief microstructures, such asself-organization in copolymer or dewetting, laser ablation, electron-or ion beam lithography and nanoimprint lithography. The microstructurescan, for example, simply be replicated by embossing using an embossingtool containing the microstructure.

A preferred method of manufacturing non-periodic, anisotropic surfacerelief microstructures is described in the international patentapplication WO-01/29148, the content of which is incorporated herein byreference. The method makes use of the so called monomer corrugation(MC) technology. It relies on the fact that phase separation of specialmixtures or blends applied to a substrate is induced by crosslinking,for instance by exposure to ultraviolet radiation. The subsequentremoval of non-crosslinked components leaves a structure with a specificsurface topography. The term MC-layer is used for layers preparedaccording to this technology. Anisotropy of the microstructure can, forexample, be achieved if liquid crystalline mixtures are used, which arealigned by an underlying alignment layer. By using an alignment layerwith an orientation pattern, it is possible to create a patterned,non-periodic, anisotropic surface relief microstructures.

In preferred embodiments an element according to the invention generatescolors when illuminated with white light. Manufacturing of suitablesurface relief microstructures is disclosed in WO2007/131375, thecontent of which is incorporated herein by reference.

Preferably, the method of manufacturing a non-periodic, anisotropicsurface relief microstructures according to the invention comprises thesteps of coating a thin photo-alignment film on a substrate, generationof an orientation pattern by exposing individual areas of thephoto-alignment film to linearly polarized UV light of differentpolarization directions, coating a blend of crosslinkable andnon-crosslinkable liquid crystal materials on top of the photo-alignmentfilm, cross-linking the liquid crystalline blend and removing thenon-cross-linked material, for example using an adequate solvent.

Cross-linking of the liquid crystalline blend is preferable done byexposure to actinic light. The cross-linking process induces a phaseseparation and cross-linking of the liquid crystal prepolymer. The basicprinciples and the optical behavior of micro-corrugated thin-films arefor example disclosed in the international patent applicationWO01/29148.

Preferably, optical elements according to the invention are at leastpartially reflective. The optical elements according to the inventiontherefore preferably comprise reflective or partially reflective layersusing materials such as gold, silver, copper, aluminum, chromium orpigments. The reflective or partially reflective layers may further bestructured such that they cover only part of the optical element. Thiscan be achieved, for example, by structured deposition of the layer orby local de-metallization.

Reflection can also be caused by a transition to a material having adifferent index of refraction. Therefore, in a preferred embodiment ofthe invention the surface of the microstructure of an optical elementaccording to the invention is at least partially covered with adielectric material. Examples of high index refraction materials areZnS, ZnSe, ITO or TiO2. Composite materials including nanoparticles ofhigh index refraction materials could also be suitable. The cover mediummay also be absorptive for certain colors to change the color appearanceof the device.

Optionally, the surface relief microstructures of an optical elementaccording to the invention may be sealed in order to protect the elementagainst mechanical impact, contamination and in order to preventunauthorized and illegal making of replicas of such elements. Therefore,optical elements according to the invention preferably comprise asealing layer on top of the microstructure.

Optical elements produced according to the present invention can be usedin different applications which deal with spatial modulation of thelight intensity. Preferably the optical elements according to theinvention are used as security elements in security devices.Specifically such security devices are applied to or are incorporatedinto documents, passports, licenses, stocks and bonds, coupons, cheques,certificates, credit cards, banknotes, tickets etc. againstcounterfeiting and falsification. The security devices further can alsobe applied as or incorporated into brand or product protection devices,or into means for packaging, like wrapping paper, packaging boxes,envelopes etc. Advantageously, the security device may take the form ofa tag, security strip, label, fiber, thread, laminate or patch etc.

EXAMPLES Sample Preparation

For the examples below the samples are made by first preparing aphoto-alignment layer on a silicon wafer by spin-coating. An orientationpattern is created in the photo-alignment layer by sequential exposureof the different zones to linearly polarized uv-light of the desiredpolarization directions, which is provided by a laser scanning exposuresystem. The laser scanning system is controlled by a computer whichprovides the pattern information of the zones of different orientationdirection. This avoids the use of multiple photo-masks and thereforeallows fast generation of arbitrary orientation patterns. The patterncomprising randomly distributed zones corresponding to the differentorientation directions with a desired area fraction is generated by acomputer algorithm.

A patterned anisotropic surface relief microstructure is thenmanufactured using the MC-technology procedure disclosed in WO01/29148,which is incorporated herein by reference.

FIG. 4 shows a photograph of a silicon wafer with four differentregions, each of them comprising a patterned anisotropic surface reliefmicrostructure with information encoded in the pattern according to theinvention.

The scattering characteristics of the samples of the examples below aremeasured by conoscopy using an ELDIM EZContrast 160R measurement system.The samples are illuminated with collimated white light from a normalincidence angle with regard to the surface plane of the sample and theluminosity of the reflected light is measured as a function of theazimuthal (0°-360°)and zenithal (0°-80°) angle.

FIG. 5 illustrates the measurement geometry, wherein the azimuthal angleis referred to as θ and the polar angle is referred to as φ.

The evaluation can either take into account the full distribution of thescattered light or it may use only part of the spatial lightdistribution if this part is characteristic and provides sufficient datato evaluate the encoded information. The evaluation of the samples ofthe examples below has been made by only taking into account the datameasured for a constant polar angle φ of 30° as indicated in FIG. 5 bythe numeral 25.

Example 1

An optical element according to the invention is made as describedabove. The element comprises a patterned anisotropic surface reliefmicrostructure with two types of regularly distributed zones, whichdiffer in the anisotropy direction. The different zones are arranged incheckerboard pattern, wherein the zones corresponding to the differentdirections are represented as black and white, respectively, as shown inFIG. 6a . Each of the zones is 50 μm×50 μm. The orientation direction inthe first type of zone is oriented at 0° with regard to a referencedirection and the first zones have a fraction f₁=50%. The orientationdirection in the second type of zone is oriented at 90° with regard to areference direction and the zones have a fraction f₂=50%.

FIG. 6b shows the result of the conoscopic measurement. Fordetermination the fraction of the different zones a curve is createdusing only the data for the polar angle 30°, as described above. For thefitting algorithm it is assumed that the scattering profiles related tothe different types of zones have a Gaussian distribution. The resultfrom the evaluation of the sample is that the fraction f₁ of zones 1 is48.5% and fraction f₂ of zones 2 is 51.5%. The result is very close tothe fraction that has been defined by the distribution pattern. Hencethe information encoded in the pattern is decoded.

Example 2

An optical element according to the invention is made as describedabove. The element comprises a patterned anisotropic surface reliefmicrostructure with three types of randomly distributed zones, whichdiffer in the anisotropy direction. FIG. 7a shows the pattern that hasbeen created, wherein the zones corresponding to the differentdirections are represented in three different grey levels. The size ofthe smallest zone (unit zone) is 50 μm×50 μm, but because of the randomarrangement larger zones are formed as well. The unit zone has the formof a square. The orientation direction in the first type of zone isoriented at 0° with regard to a reference direction and the first zoneshave a fraction f₁=50%. The orientation direction in the second type ofzone is oriented at 45° with regard to a reference direction and thezones have a fraction f₂=30%. The orientation direction in the thirdtype of zone is oriented at 90° with regard to a reference direction andthe zones have a fraction f₃=20%.

FIG. 7b shows the result of the conoscopic measurement. The evaluationis made as described in example 1. The result from the evaluation of thesample is that the fraction f₁ of zones 1 is 52.5%, fraction f₂ of zones2 is 33.4% and fraction f₃ of zones 3 is 14.1%. The result is very closeto the fraction that has been defined by the distribution pattern. Hencethe information encoded in the pattern is decoded.

Example 3

An optical element according to the invention is made as describedabove. The element comprises a patterned anisotropic surface reliefmicrostructure with three types of randomly distributed zones, whichdiffer in the anisotropy direction. FIG. 8a shows the pattern that hasbeen created, wherein the zones corresponding to the differentdirections are represented in three different grey levels. The size ofthe smallest zone (unit zone) is 50 μm×50 μm, but because of the randomarrangement larger zones are formed as well. The unit zone has the formof a square. The orientation direction in the first type of zone isoriented at 0° with regard to a reference direction and the first zoneshave a fraction f₁=33.3%. The orientation direction in the second typeof zone is oriented at 45° with regard to a reference direction and thezones have a fraction f₂=33.3%. The orientation direction in the thirdtype of zone is oriented at 90° with regard to a reference direction andthe zones have a fraction f₃=33.3%.

FIG. 8b shows the result of the conoscopic measurement. The evaluationis made as described in example 1. The result from the evaluation of thesample is that the fraction f₁ of zones 1 is 34.1%, fraction f₂ of zones2 is 35.0% and fraction f₃ of zones 3 is 30.9%. The result is very closeto the fraction that has been defined by the distribution pattern. Hencethe information encoded in the pattern is decoded.

Example 4

An optical element according to the invention is made as describedabove. The element comprises a patterned anisotropic surface reliefmicrostructure with three types of randomly distributed zones, whichdiffer in the anisotropy direction. FIG. 9a shows the pattern that hasbeen created, wherein the zones corresponding to the differentdirections are represented in three different grey levels. The unit zonehas the form of a triangle. The orientation direction in the first typeof zone is oriented at 0° with regard to a reference direction and thefirst zones have a fraction f₁=33.3%. The orientation direction in thesecond type of zone is oriented at 60° with regard to a referencedirection and the zones have a fraction f₂=33.3%. The orientationdirection in the third type of zone is oriented at 120° with regard to areference direction and the zones have a fraction f₃=33.3%.

FIG. 9b shows the result of the conoscopic measurement. The evaluationis made as described in example 1. The result from the evaluation of thesample is that the fraction f₁ of zones 1 is 33.2%, fraction f₂ of zones2 is 33.1% and fraction f₃ of zones 3 is 33.7%. The result is very closeto the fraction that has been defined by the distribution pattern. Hencethe information encoded in the pattern is decoded.

1. An optical element comprising: a region with a patterned anisotropicsurface relief microstructure, which has zones with different anisotropydirections θ_(i), wherein information is encoded in the distribution ofzones with the different anisotropy directions.
 2. The optical elementaccording to claim 1, wherein the information is encoded by the fractionof the areas of zones with the anisotropy direction along θ_(i) withregard to the total area with anisotropic surface relief microstructurein that region.
 3. The optical element according to claim 1, wherein thepattern is combined with an OVD feature.
 4. A method of using an elementaccording to claim 1 comprising: applying the element to banknotes orother value documents and using the element as a security feature in thebanknotes or other value documents.
 5. A method for evaluating theinformation encoded in the orientation distribution of an elementaccording to claim 1, comprising detection of the spatial lightdistribution scattered from an element comprising a region with apatterned anisotropic surface relief microstructure, which has zoneswith different anisotropy directions θ_(i), wherein information isencoded in the distribution of zones with the different anisotropydirections, and evaluation of the fractions f_(i)(θ_(i)) from themeasured spatial light distribution, wherein f_(i)(θ_(i)) is thefraction of the areas of zones with the anisotropy direction along θ_(i)with regard to the total area with anisotropic surface reliefmicrostructure in that region.
 6. The method according to claim 5,wherein the evaluation of the information encoded in the orientationdistribution comprises making a measurement of the spatial lightdistribution using a conoscopy imaging system.
 7. The method formanufacturing of an optical element according to claim 1, comprising useof an encoding algorithm to generate a pattern of an orientationdistribution, which represents the information to be encoded, andmanufacturing of a patterned anisotropic surface relief microstructure,which has zones with different anisotropy directions θ_(i), according tothe pattern that has been calculated.